Pixels used in conventional image sensors are generally classified as 3-transistor pixels, 4-transistor pixels, or 5-transistor pixels according to the number of transistors included therein.
FIGS. 1 to 3 show a typical pixel structure used for an image sensor, according to the number of transistors;
FIG. 1 shows a 3-transistor structure. FIGS. 2 and 3 show a 4-transistor structure.
As shown in FIGS. 1 to 3, a fill factor that is the area occupied by the photodiode over the entire area of the pixel is naturally reduced due to the existence of transistors in a pixel circuit. In general, the fill factor of a diode ranges from 20 to 45%, considering capability of each semiconductor manufacturing process. Accordingly, light that is incident onto the rest area corresponding to about 55-80% of the entire area of the pixel is lost.
To minimize the loss of optical data, a microlens is used for each unit pixel in a manufacturing process of the image sensor so that the optical data can be condensed onto the photodiode of each pixel. A microlens gain is defined as an increment of the sensitivity of a sensor using the microlens with respect to the sensitivity of the image sensor without the microlens.
Given that the fill factor of a common diode is about 30's %, the microlens gain is 2.5-2.8 times of the sensitivity of the image sensor without the microlens. However, a pixel size has decreased to 4 μm×4 μm, and even to 3 μm×3 μm. Further, with an emergence of a small-sized pixel of 2.8 μm×2.8 μm or 2.5 μm×2.5 μm, starting from when the pixel size is 3.4 μm×3.4 μm, the microlens gain significantly drops from 2.8 times to 1.2 times of the sensitivity of the image sensor without the microlens. This is caused by diffraction phenomenon of the microlens. The severity of diffraction phenomenon is determined by a function of a pixel size and a position of the microlens.
As the pixel size gradually decreases, the severity of diffraction phenomenon of the microlens increases, thereby dropping the microlens gain equal to or less than 1.2 times of the sensitivity of the image sensor, which results in a phenomenon where the light condensation seems to be unavailable. This is newly being recognized as a cause of sensitivity deterioration.
In general, the decrease of the pixel size of the image sensor results in the decrease of the area of the photodiode. The area of the photodiode is closely related to the amount of available electric charge of the photodiode. Accordingly, the amount of available electric charge decreases when the size of the photodiode decreases. The amount of available charge of the photodiode is a basic factor for determining a dynamic range of the image sensor, and therefore the decrease of the amount of available electric charge directly affects the image quality of the sensor. When the image sensor of which the pixel size is less than 3.2 μm×3.2 μm is manufactured, its sensitivity decreases, and the dynamic range of the sensor with respect to the light also decreases, thereby deteriorating the image quality.
An external lens is used in the process of manufacturing a camera module using the image sensor. In this case, light is substantially perpendicularly incident onto a center portion of a pixel array. However, the light is less perpendicularly incident onto edge portions of the pixel array. When an angle starts to deviate from the vertical angle by a predetermined degree, the light is condensed onto the microlens which is out of the area pre-assigned for condensation of the photodiode. This generates a dark image, and more seriously, when the light is condensed onto a photodiode of an adjacent pixel, chromaticity may change.
Recently, with the development of the image sensor having from 0.3 million pixels and 1.3 million pixel to 2 million pixels and 3 million pixels, a dynamic zoom-in/zoom-out function as well as an automatic focus function are expected to be included in a compact camera module.
The features of the functions lie in that the incident angle of the light significantly changes at edge portions while each function is performed. The chromaticity or brightness of the sensor has to be independent of changes in the incident angle. With the decrease of the pixel size, the sensor cannot cope with the changes in the incident angle. At present, the sensor can support the automatic focus function, but the sensor can not support the dynamic zoom-in/zoom-out function. Therefore, it is difficult to develop a compact camera module providing a zoom function.